This invention relates to nuclear magnetic resonance (NMR) imaging and, more particularly, to an improved method for enabling high-spatial-resolution spectroscopic NMR imaging with selected nuclei of a sample containing nuclei having chemically-shifted NMR frequencies.
As is well known, the nuclear magnetic resonance phenomenon is exhibited by atomic nuclei with an odd number of either protons or neutrons. Such nuclei possess spin, which endows them with a small magnetic field. When placed in an externally applied static main magnetic field B.sub.o, the nuclei tend to align themselves with the applied field and produce a net magnetization M in the direction of the applied field. The nuclei oscillate, or precess, about the axis of the applied field with a characteristic NMR frequency, .omega..sub.o, given by the Larmor equation: EQU .omega..sub.o =.gamma.B.sub.o ( 1)
where .gamma. is the gyromagnetic ratio and is constant for each NMR isotope. The NMR precession frequency is directly proportional to the applied field B.sub.o. If a time-dependent radio-frequency (RF) magnetic field, having a frequency component equal to the Larmor frequency of the nuclei, is applied in a direction orthogonal to the main field, then the nuclei will absorb energy and nutate away from the axis of the main field and commence to precess at the Larmor frequency about the new net applied field direction. When the RF field is turned off, the nuclei emit NMR signals at their characteristic Larmor frequency, which signals decay as the nuclei relax or return to equilibrium in alignment with the main field. These NMR signals may be detected and Fourier transformed to derive the frequency components of the NMR signals which are characteristic of the nuclei.
Nuclei of the same isotope can exhibit minute variations in their NMR frequencies, which are referred to as chemical shifts, because of differences in their chemical environments which cause differences in their magnetic field environments. Chemical shifts result from alterations of the magnetic field around nuclei as a result of the shielding currents that are associated with the distribution of electrons around adjacent atoms. The degree of shielding is characteristic of the environment of the nucleus, and thus the chemical-shift spectrum of a given molecule is unique and can be used for identification. In conventional NMR spectroscopy, the chemical structure of the sample is studied by observing the chemically-shifted signals returned from an NMR experiment. Because the resonant frequency and the absolute chemical shift are dependent upon the strength of the field, the chemical shift is expressed as a fractional shift in parts-per-million (ppm) of the resonant frequency relative to an arbitrary reference compound.
Since the Larmor frequency is proportional to the magnetic field, if the magnetic field varies spatially in a sample, then so does the resonant frequency of the nuclei. In NMR imaging, one or more magnetic field gradients are applied to the sample to spatially encode the emitted NMR signals. By applying an RF excitation pulse (having a narrow range of frequency components) to the sample in the presence of gradients, the nuclei in a selected region, e.g., a planar slice, or at a selected point, of the sample can be selectively excited and their NMR response signals detected. The data collected from different regions or points the sample can be processed in a well-known manner to construct an image.
NMR imaging in the past has typically been performed in rather low magnetic fields and chemical shifts have not been a significant problem. In magnetic fields below about 0.7 Tesla (T), chemical shifts are difficult to observe because of the natural linewidths of the resonances and the low sensitivity of nuclei other than hydrogen (.sup.1 H). It is desirable, however, to perform NMR imaging in higher magnetic fields, in excess of 1 T for example, because of the improved signal-to-noise ratios realized; recent advances in magnet technology permit the use of higher magnetic fields, of the order of 1-1.5 T, in medical and biological NMR imaging. As the magnetic field increases, the chemical-shift increases proportionately and becomes a greater problem. Chemical shift can produce the same effect as a spatial variation in the NMR signal. This results in chemical shift artifacts which are manifested, for example, as ghosts in two-dimensional-Fourier-transform (2DFT) imaging. Ghost artifacts may appear as a faint ring or ghost at one side of an image, and such ghosts both obliterate some of the spatial information present and reduce spatial resolution.
In proton imaging of the body, the chemical shift observed is principally between the hydrogens attached to oxygen in water and the hydrogens attached to carbon in CH.sub.2 lipid (fat) tissue; this chemical shift is on the order of 3 ppm. The effect of the chemical shift is to produce two superimposed images; one image is the water image and the other image is the lipid image, which is shifted along the axis on which the projection was made by an amount corresponding to the chemical shift. At magnetic field strengths on the order of B.sub.o =1.5 T, for example, the chemical shift results in an artifact in the NMR image and is a significant problem.
It is desirable to provide NMR imaging methods that permit separation of superimposed chemically-shifted images, such as those of an aqueous proton image and a lipid proton image, into distinct images so as to remove chemical-shift artifacts and improve the spatial resolution in the images. Separate resolved images are also desirable for other reasons. For example, an image constructed from lipid protons alone may be useful for looking at fat or atherosclerotic lesions or plaques in blood vessels, as well as for the evaluation of heart disease.
One approach which may be useful for separating the images is resolved-spectroscopy, which employs selective slice excitation, pulse gradient (phase) encoding in both x and y directions (to avoid blurring out the spectroscopic information by having a gradient on when the free induction decay (FID) is collected), and finally a Fourier transformation from the time domain to the frequency domain for each point of the imaging plane, i.e., at each pixel of the display. For an image array comprising N-by-N pixels, this approach requires N.sup.2 projections. For example, a 256-by-256 array would require 65,536 projections to produce two resolved images. The amount of time required for this number of projections would be unreasonable, making a resolved-spectroscopy approach impractical.
In co-pending application Ser. No. 565,305, filed Dec. 27, 1983, U.S. Pat. No. 4,581,582, assigned to the same assignee of the present application and incorporated herein in its entirety by reference, one of the present inventors describes and claims a method of NMR imaging that enables superimposed images, produced by chemically-shifted nuclei and formed as projections along first and second lines that extend in different, preferably orthogonal, directions to be resolved and separated into distinct chemical-shift images. EAch NMR image contains first and second image components respectively produced by first and second chemically-shifted nuclei in the sampled region. After one of the pair of NMR images is registered with the other image by registering the first image components, the two images are combined to subtractively eliminate the first image components and to produce a composite image containing the second image components. This process requires that the same degree of imaging linearity be obtained along both projection lines, which are typically in orthogonal directions. Since all gradient magnetic fields (used to establish the lines of projection) are less than perfectly linear in real-world imaging apparatus, it is extremely difficult to provide identical spatial and temporal distortion along the substantially orthogonal projection axes; any non-identical distortion results in the undesired effects, even though such effects are reduced in scope.